The present invention relates to fabrication of MOS transistors with reduced susceptibility to channel hot carrier effects.
Hydrogen passivation of the dangling bonds which form electrically active interface traps improves device function, but hydrogen passivation can be degraded by subsequent hot electron impact. Recent results have shown that passivation with deuterium instead of hydrogen in the post-metal anneal process is more stable and therefore less likely to succumb to hot electron degradation, resulting in an improvement in channel hot carrier lifetime of 10-100X. I.C. Kizilyalli et al., "Deuterium Post-Metal Annealing of MOSFET's for Improved Hot Carrier Reliability," 18 IEEE Electron Device Letters, 81 (1997); J. W. Lyding et al., "Reduction of Hot Electron Degradation in Metal Oxide Semiconductor Transistors by Deuterium Processing," 68 Appl. Phys. Lett. 18 (1996), which are hereby incorporated by reference.
However, the deuterium sinter has been found not to work as well on devices which have silicon nitride sidewall spacers. This unworkability is believed to be due to the nitride spacer acting as a post-deposition source of hydrogen. Any hydrogen incorporated during deposition may become a post-deposition hydrogen source by outdiffusion from the film during high temperature treatments such as sintering. A post-deposition hydrogen source can passivate the dangling bonds before a final deuterium sinter is able to. Additionally, silicon nitride blocks deuterium diffusion. This can be a problem when silicon nitride is used as the etch-stop layer during the formation of self-aligned contact holes. Deuterium passivation of the gate interface in such a structure proceeds very slowly.